As described in co-pending U.S. patent application Ser. No. 10/444,510, FIG. 1 illustrates two millimeter wave sources 10, 20 radiating collimated beams 12, 22 of electromagnetic radiation at two separate frequencies, f1 and f2, and in two intersecting directions that overlap at a distance. See U.S. patent application Ser. No. 10/444,510, entitled: Method And Apparatus For Directing Electromagnetic Radiation To Distant Locations, filed May 23, 2003, the contents of which are hereby incorporated by reference in its entirety. Generally, when two electromagnetic beams of different frequencies converge, the volume of the intersection, often referred to as the interference zone 24, will include a frequency component equal to the difference in frequency of the two beams, which is defined herein as the interference difference frequency, Δf. More specifically, the electromagnetic interference at the interference difference frequency, Δf, is optimal in that the electromagnetic interference field strength is at a maximum when the beams are diffraction limited and collimated having substantially equal intensities and with aligned polarizations. When the interference difference frequency is incident upon electronic components, the resultant field will interfere with the operation of the electronics.
At the interference difference frequency, Δf voltages and currents are generated by intermodulation through nonlinear surface and volume effects (such as oxide layers, corroded surfaces, etc.), also by nonlinear electronic circuit parts and components, such as diodes and transistors, which are common to integrated circuits, receiver front-ends, and other circuit parts that may resonate with either or both the main and difference frequencies that are projected. For example, when the collimated and coherent outputs of two distinct millimeter wave antennas are 100 GHz and 101 GHz and there is a nonlinear component in the interference zone, there will be a 1 GHz component created in the electrical circuits that are connected to the non-linear components. Physically, the interference pattern created in the interference zone of collimated parallel polarized beams is a fringe field where the fringe planes are parallel to one another. The fringe planes are traveling in a direction perpendicular to the planes at the rate of the interference difference frequency, i.e. difference between the frequencies. The fringe planes are separated by the fringe period, λf, which is determined by
                              λ          f                =                              λ            0                                2            ⁢                                                  ⁢            sin            ⁢                          θ              2                                                          (        1        )            where λ0 is the average wavelength of the two collimated beams, and θ is the angle of intersection between the two collimated beams. As can be seen, the fringe period depends upon the angle of intersection of the intersecting beams. Additionally, when the beams are at substantially equivalent field strengths, full amplitude modulation of the interference field will be achieved.
FIG. 2 illustrates an alternate method to converge electromagnetic beams at a distance in a special case of the converging angle θ=0. Two millimeter wave sources 30, 40 radiate collimated beams 32, 42 of electromagnetic radiation at two separate frequencies, f1 and f2, and in the direction of a polarization beam combiner 34. The polarization beam combiner combines orthogonally polarized beams by reflecting one beam and permitting transmission therethrough of the other beam. The resultant output is therefore the combined beams of both collimated beams 32, 42 having an interference difference frequency as described above. Again, for example, if f1=100 GHz and f2=101 GHz, the resultant interference difference frequency Δf=1 GHz. In contrast to the above description, however, the intersection angle, θ, between the two beams is reduced to zero. As such, the fringe period has become infinite, that is to say that there are now no fringes and no spatial variation of intensity in any plane perpendicular to the direction of beam propagation.
In a typical arrangement, the polarization beam combiner surface is oriented at 45 degrees with respect to the beams (32, 42 in FIG. 2). The polarization beam combiner 34 is arranged to transmit the linearly polarized incident beam 42 with the minimum of loss. The other beam (32 in FIG. 2) will be polarized orthogonal to the first beam to obtain maximum reflection through the polarizer. Once these two beams are combined, they are superimposed and may be directed. That is to say that both beams 32, 42 are transmitted within one effective beam rather than separate converging beams (as described in FIG. 1), and the resultant interference zone 44 is the volume occupied by the merged beams, from the polarizer and beyond.
While a linear polarization beam combiner 34 has been discussed above other embodiments of beam combiners, known to those of ordinary skill in the art, including beam splitters, circular polarization beam combiners, and the like, may be substituted accordingly. Additional information relating to superimposition of electromagnetic beams is further described in the background, above, and in co-pending U.S. patent application Ser. No. 10/444,510 incorporated herein by reference.
Having developed methods of effectively combining electromagnetic beams at distant locations, it would be desirable to utilize the difference frequency generated in these interactions. Microwave transmitters and receivers, such as those that are part of military and civilian radar systems can be identified by their active emissions. To evade detection, microwave transmitters avoid transmitting or power down entirely. In such detection avoidance circumstances, transmitters and receivers are powered up only for brief periods at a time, making them practically invisible to commonly used electronic surveillance measures. There is no presently available method to detect, locate and identify the presence of inactive microwave receivers from long ranges. Interactions created by electromagnetic interference, as described above, may aid in identification of such inactive microwave devices.
As used herein, several terms should first be defined. Microwaves are the radiation that lies in the centimeter wavelength range of the electromagnetic (EM) spectrum (in other words: 1 cm<λ<100 cm; that is, the frequency of radiation in the range between 300 MHz and 30 GHz, also known as microwave frequencies). Electromagnetic radiation having a wavelength longer then 1 meter (or frequencies lower than 300 MHz) will be called “Radio Waves” or just “Radio Frequency” (RF). For simplicity in this disclosure, the RF spectrum is considered to cover all frequencies between DC (0 Hz) and 300 MHz. Millimeter Waves are the radiation that lies in the range of frequencies from 30 GHz to 300 GHz, where the radiation's wavelength lies in the 1 mm<λ<10 mm range. Finally, electromagnetic frequencies from 300 GHz to 3 THz are described as submillimeter waves, but on some occasions are often lumped with millimeter waves. As known to those of ordinary skill in the art, for practical purposes the “boundaries” for these above four frequency ranges are often not precisely observed. For example, a cell phone antenna (and its circuitry operating in the 2.5+GHz range) is associated with RF terminology and considered as part of RF engineering. A waveguide component for example, covering the Ka band at a frequency around 35 GHz is usually called a microwave (and not a millimeter wave) component, etc. Accordingly, these terms are used for purposes of consistently describing the invention, but it will be understood to one of ordinary skill in the art that alternative nomenclatures may be used in more or less consistent manners.